I've seen the Enterprise turned around in as little as 10 minutes in Connolly, but I presume that's under exceptional circumstances. It's one thing to do quick turnarounds on an occasional basis, but if they have to turn around that quick all day every day, there isn't time for the routine checks and servicing, not to mention cleaning. Things will inevitably start to break down, and the quality of service will degrade.
Aside from that, having the service split between two different train types causes a lot of complexity for maintenance, servicing and training, and also makes online seat booking difficult (what happens if a Mk4 is scheduled to run a service and it gets swapped for a DD at the last minute, and the seat number you've reserved doesn't exist on the replacement train?).
I hope we'll see an hourly Enterprise service one day, but it will probably need to coincide with the replacement of the DDs by new stock.